Various electro-mechanical sensors exist used for measuring angular position between two bodies rotating with respect to each other. However, such sensors usually measure the relative angular position of bodies mechanically connected by rigid joints, for example the steering wheel and the transmission shaft of a car.
The nature of the sensors mentioned above remarkably limits the applicability in special fields such as, for example, virtual reality or the design of artificial prosthesis in medicine.
In particular, such sensors are not suitable in fields that, owing to technological progress, have recently captured the attention of many researchers, i.e. acquisition and analysis of movement of human body parts. By using absolute position data of single points of the human body or angular relative positions between two adjacent limbs, a digital model of the human body can be created.
The known sensors suitable for this particular fields depend on the type of application and on the limbs that have to be monitored.
In particular, the limbs with greater volume, such as arms and legs of a human, have few degrees of freedom and the devices used for detecting their motion usually have higher weight and encumbrance and require a higher rate of precision. The limbs with smaller volume, instead, such as the fingers of the hand that have a higher number of degrees of freedom, cannot be monitored with heavy and bulky devices and require less precision.
The known sensors used for tracking the motion of the limbs of the human body are usually of magnetic, optical, ultrasonic or mechanical nature.
Among them, two main categories of sensors exist:                1) sensors that do not require a mechanical connection between the reference object and the object to be monitored.        2) sensors that require a mechanical connection between the reference object and the object to be monitored.        
To the first category the following belong:
Magnetic sensors, which require one or more transmitters for creating a magnetic field in a determined workspace. In particular, they have high costs and have the further drawback of being particularly sensitive to the presence of metal that can distort the magnetic field.
Optical sensors, which require optical markers, either active or reflective, whose light is captured allowing a computer to calculate its position. Usually, the optical devices are less bulky than the magnetic, but their functionality can be affected by parts of the body that cross accidentally the path of the light. Another drawback of the optical devices are high costs and the need for a post-processing of the measured data, as well as the long set up time for calibrating the measuring equipment.
To the second category the following belong:
Sensors of mechanical type, based on strain gauges embedded in a flexible support structure, providing the angle of rotation of the sensor-support assembly. Their performances are high but they have reliability problems and a high cost;
Sensors based on optical lines, which measure the variation of intensity of a light beam projected in an optical line. This measure reveals the angle of rotation of the sensor support. Such sensors have a relatively low cost but have reliability problems and low measuring precision.